Carl Wunsch, The Economist and the Gulf Stream

Carl Wunsch usually has very interesting things to say about the climate system, and although his arguments don’t necessarily win everyone completely over, they often generate an improvement in the level of scientific discussion. In this week’s Economist, he has a letter printed concerning the mis-definition of the ‘Gulf Stream’ concept in the magazine’s climate change survey a couple of weeks ago. This is essentially a reprint of his letter to Nature that was published in 2004, which stated correctly that the Gulf Stream is basically a wind driven phenomenon and will not stop or reverse while the wind still blows and the Earth still turns.

The offending Economist statement was ‘The Gulf Stream is driven both by the rotation of the Earth and by a deep water current called the thermohaline circulation’ in an article discussing the likelihood of a ‘shutdown of the Gulf Stream’. Senso stricto, Wunsch is absolutely correct; the Gulf Stream in oceanographic terms refers to the very strongly intensified current on the western boundary of the Atlantic running from Florida to the Carolinas and which heads off into the mid-Atlantic at Cape Hatteras (see figure). These kinds of currents appear on the western boundaries of basins everywhere in the mid-latitudes and arise from the basic pattern of the winds (easterlies in the tropics, westerlies in the mid latitudes) and the rotation of the Earth (they do also require some kind of rotational gradient like you get on a spherical Earth – they wouldn’t exist on a cylindrical rotating planet, for instance – look up the ‘beta effect‘ if you are interested).

However, the Economist is using the term in a much more colliquial (and common) sense that conflates this current with the Meridional Overturning Circulation (MOC, often conflated with the Thermohaline Circulation) which involves convection in the waters around Greenland and the deep currents that cool the deep ocean. This use of the term is often synomymous with northward ocean heat transport (the North Atlantic Current) that contributes to Europe’s warmth and which have often been fingered as a particularly sensitive aspect of the climate. While in one sense the water flow associated with the MOC does contribute to the Gulf Stream, it is definitely the junior partner, and so any changes in the MOC are not going to threaten the Gulf Stream in any existential way. However, a shutdown in the MOC does not make as good a headline as a shutdown in the Gulf Stream, and so this misuse persists in the media and public alike (though not in The Day After Tomorrow – they used ‘North Atlantic Current’ throughout!).

If the definition of Gulf Stream was really all that this was about, I doubt Wunsch would have picked up his pen, however, what Wunsch really objects to is the casual use of the word ‘driven’. This is a much more subtle point and one which even the scientific community hasn’t fully assimilated yet. There is a standard theorem of oceanography called Sandström’s theorem which basically states that it is really hard to do any work on a system, like the ocean, if you are cooling and heating at the same level (i.e. the surface). By contrast, the atmosphere is heated from below (since the atmopshere is mostly transparent to solar radiation), and cooled from above and so can behave as a relatively efficient heat engine. Since we observe however that the ocean does have a large-scale deep circulation, it can’t therefore be ‘driven’ (in an energetic sense) from the cooling of waters in the high latitudes. The alternate name, ‘the Thermohaline Circulation’, does tend to perpetuate the idea that it is the temperature and salinity fluxes that ‘drive’ the circulation, and so is slowly falling into disfavour (though it is still widely used and defended).

But if this deep circulation doesn’t derive its energy from density contrasts, where does it get the energy from? Most of the energy in the oceans is derived from two sources, the winds and the tides. Both of these forces generate small scale turbulence and internal waves which cause mixing of ocean waters. It is this mixing which energetically fuels the deep ocean circulation.

Since the winds will continue to blow and the Earth continue to turn, does this mean that there can’t be any changes to the MOC? Emphatically no. The circulation may well derive it’s energy from the winds and tides, but it is heavily steered by density contrasts and the stratification of the ocean (witness the difference between the North Pacific and the North Atlantic). Changes in that modulation can have profound effects on the currents, and in particular, additions of fresh water from massive lake drainages (i.e. the 8.2 kyr event) or ice sheet collapses (the Heinrich events) most likely caused severe slowdowns or shutdowns of the MOC in the past. Wunsch is a little sceptical of this research (he calls fresh water the ‘deus ex machina’ of climate change), but in this he is probably mistaken – for instance, there is enough information from the 8.2 kyr event to reasonably attribute it to the drainage of Lake Agassiz into Hudson Bay.

Thus while density changes don’t ‘drive’ the circulation (in an energetic sense) they can ‘drive’ (in a modulating sense) changes in that circulation. If this seems complicated, think of the example of greenhouse gases – they don’t drive the climate in an energetic sense (the sun does), but they can drive changes in the climate (by modulating radiation flow in the atmosphere).

That isn’t to say that a future MOC shutdown is either likely or imminent, but see our previous posts for reasons to be wary of drawing conclusions from current observational data.

In summary, these definitional issues may seem like pendantry at times, but there is often a more subtle point of understanding at issue. It would be nice if everyone used words in the exact same way, but until that happy utopia dawns, letters like Wunsch’s are probably needed every so often to keep us all on the straight and narrow.

Update: Stefan rightly notes below that that the term ‘Thermohaline Circulation’ has a long history of applying similarly ‘driven’ flows and is not strictly synonymous with the MOC. His paper on the subject is worth reading.

32 Responses to “Carl Wunsch, The Economist and the Gulf Stream”

As a graduate student still learning much of the terminology used in climate change science, I often find myself doing google searches for the definition of some terms. Wunsch’s commentary highlights the frustration I often have with terms used incorrectly.
A useful paper for myself, and perhaps one to be repeated for climate change science, is “Suggested Terminology for Quaternary Dating Methods” (Colman et al., Quat. Res., 28, 314-319, 1987). It discusses the misuse of terms and makes suggested recommendations. A similar article addressing commonly misused terms in climate change science could aid the science researcher, especially the younger generation. Perhaps such a publication would take us one step closer to the “utopia” mentioned above.

Clarification required–the excellent summary by Gavin first refers to the western edge of the Atlantic, then the western edge of the “continent”–I am sure he wasn’t meaning to discuss the California Current system, although the Snyder et al. paper in GRL in 2003 is interesting from an RCM point of view.

[Response: My bad. I’ve changed it above, it should be western edge of the basin. – gavin]

Aside from potentially mitigating warming in Europe, wouldn’t a slow down or even shut down of the North Atlantic Current effect the carbon cycle? ie less ocean mixing => less CO2 removed from the air.

I have also wondered about what effect a change in the THC would have on temperatures in the deep ocean. If cold arctic water is no longer sinking would temperatures rise at the bottom?

How much does thermal expansion in the tropics contribute to ocean circulation?

Obviously, hot tropical water expands. The greater density of cool water north and south of the tropics would squeeze the warmer water up where it would then flow north and south on the surface. Cool replacement water would push into the tropics at some level below the surface. Wouldn’t this ripple across the whole ocean? Or is evaporation increasing the saline concentration making the hot water denser and thus countering the effect?

Gavin – this is great, but I’m unclear on one point,Most of the energy in the oceans is derived from two sources, the winds and the tides. Both of these forces generate small scale turbulence and internal waves which cause mixing of ocean waters. It is this mixing which energetically fuels the deep ocean circulation.
Small scale turbulence sounds like a great way to dissipate energy, not drive a large scale circulation. Can you elaborate?

[Response: Well, it’s not terribly efficient… The mixing caused by turbulence and breaking internal waves and the like causes heat to be mixed down into the ocean and increases the potential energy of the system. This potential energy can then be used to drive the large scale overturning. In terms of Sandström’s theorem, it’s like you now have a source of heat below the surface and therefore you can drive a circulation. – gavin]

Great article. I was just wondering, when you say that many people conflate the THC and the MOC, do you mean that these are two separate phenomena or do you mean that MOC is simply a more precise name for what is sometimes referred to as the THC?
While this is my first comment, I’ve been reading the site for a while and I really enjoy it.

I was grinding my teeth over Wunsch’s letter. Thanks, Gavin, for saying what I would have said about the popular and oceanographic senses of the term Gulf Stream and of its northern offshoot.

Apropos “driven,” one has to consider the other end of the loop as well, the upwelling, as a driver. I seem to recall Stefan and Ray explaining to me, some years ago over wine, that the furious westerlies in the southern oceans provide an Ekman effect and thus an uncapping and upwelling, that this might be the real power behind the south-to-north Atlantic system that goes beyond the symmetrical MOC aspect.

The density contrast in the deep ocean can be likened to the density contrast in an estuary. Buoyancy is input at the surface (via heating and river influences respectively). On its own, this buoyancy input creates only a small pressure gradient and therefore a small circulation. However, turn the vertical mixing up, and you have a warmer (or fresher) deep pool of water under the buoyancy source. This considerably raises the potential energy of the system, which slumps, and drives a much larger circulation than if the mixing was not present.

As you say, most of the energy that goes into small-scale turbulence simply dissipates (only about 20% goes into raising the potential energy). Therefore you need an an external source of energy to provide the turbulence and mixing. In the case of the ocean, we believe it is the wind and the tides. As you intuit, the buoyancy-driven flow cannot bootstrap itself into increasing it’s strength.

One thing that I have wondered about is why the deep ocean is so cold? I would think that geothermal heating would be a constant heat source “from below.” (When you go a mile down in the earth it is much warmer than at the surface – but not with the oceans.) So circulation must be a much bigger effect that simple heat conduction. Even so – the only source for all of that cold water can be the polar regions. The earth’s average surface temperature is warmer than the ocean depths. So wouldn’t geothermal heating “from below” be a source of energy to fuel circulation of these deep ocean currents?

I think I know this one. The water down there *is* geothermally heated, but then it rises as heat rises. The cold water from above falls constantly. Even the higher pressure (which ‘should’ be increasing the temp) isn’t enough to overcome the lack of solar heating at those depths. And the evaporation at the surface is what causes the waters to cool, in general.

Now, for a circulation effect, THC is pretty powerful, and overcomes much of the vertical mixing, except at particular points like south of Greenland, south of India, and south of Alaska.

I’m no climate scientist, but I will step in here in defence of Prof. Wunsch; he has a point. In his paper/speech of March this year, posted on RC by Bryan Sralla, Wunsch explains that the ‘popular’ ( and this also means popular in climate science, too) notion (and associated graphics) of the THC is so generalised as to render it false, in the sense that it gives a false picture of the complexity of the processes, and the causal agency of the processes. He also points out that, as the idea of a ‘shutdown’ of the THC is dependent on the ‘continuous loop’, or ‘conveyor analogy’ of the ocean circulation, this hypothesis is not tenable.

With respect to the 8.2 Kyr event; I believe this was Broecker’s theory in the nineties; I understand he has recently ‘stepped back’ from the hypothesis that the event was caused by a freshwater ‘hosing’ event. (I may be wrong on this, though).

There have been plenty of recent papers (e.g. Curry & Mauritzen, 2005) which have analysed the North Atlantic in detail, and have found to date no observable change in the ocean circulation. In January this year, a Norwegian report on the NwAC over the past 10 years showed a warming, but slowing surface current – a change – but no net flux in heat transport over the period.

I am not saying the Wunsch is correct in everything that he says, but I do think that, collectively, we may sometimes take ‘depictions’ like the THC for granted, and that we shouldn’t. Therefore I praise the good professor for drawing our attention to this.
Regards,

—excerpt—
… Eric Kunze and John Dower, at the School of Earth and Ocean Sciences at the University of Victoria in British Columbia, Canada, and colleagues have measured the turbulence generated at dusk by krill as they rise to the surface of the ocean to feed.
…
During the day krill hide about 100 metres below the surface. As night falls they ascend in vast numbers. In the Saanich Inlet…, for example, krill occur in concentrations of up to 10,000 per cubic metre ….

… a microstructure profiler … measures shear forces, temperature and conductivity on a microscale, allowing the scientists to determine the amount of turbulence. “Shear sensors are like old phonograph needles that are very sensitive to bending” …

“… the layer of krill came up and we immediately saw that the turbulence levels were higher.”

And they were much higher: the krill elevated the turbulence by 3 to 4 orders of magnitude, and increased mixing between water layers by a factor of 100.

“One of the reasons we care about mixing is that surface layers mix with atmospheric gases,” says Dower. “This could play a role in the ‘drawing down’ of carbon dioxide from the atmosphere into the ocean.”

The above paper provides evidence for massive cooling of the earth’s ocean 2003 to 2005, I would assume due to changes in the earth’s upper atmosphere.

“Recent (2003 to 2005) Cooling (3.2 x 10^22 J) of the Upper Ocean”

Published 20 September 2006, Geophysical Research Letters

“We observe a net loss of 3.2 (+/- 1.1) x 10^22 J of heat from the upper ocean between 2003 and 2005. Using a broad array of in situ ocean measurements …We have detected a new cooling event that began in 2003 and is comparable to the one in the early 1980’s (6 x 10^22J lost from 1980 to 1983).”

“Assuming the 3.2×10^22J was not lost to the deep ocean, (believed not to be possible in the short time interval), previous work suggests that the scale of the heat loss is too large to be stored in any single component of the Earth’s climate system (Levitus et al 2005). A likely source of the cooling is a small net imbalance in the 340 W/m^2 of radiation that the earth exchanges with space. Imbalances in the radiation budget of the order of 1 W/m^2 have been shown to occur on these time scales … (Wong et al 2006).

The observed rapid periodic (200yr, 500yr, 1500yr) drops in the climatic record (5C to 20C drop in temperature on the Greenland Ice Sheet. Also observed in the Southern Hemisphere but response is less due to land mass distribution and cryosphere differences between the two hemispheres), were and are still believed by some, to be due to changes in the thermohaline conveyor. Seager et al, disproved that hypothesis, and notes in his paper that his quantitative conclusion that a complete stoppage of the thermohaline conveyor only result in a 1C to 2C cooling of Europe, is supported by publish work of others and simple fundamental climatic facts and analysis. The following is a link to Seager’s paper.

Excellent article, professor Wunsch brings out what I have learned by watching daily Global Weather animation maps, there are particular locations on Earth which seem, by illusion, to give birth to Cyclones, one of my favorite spot s is next to Southeast Greenland where massive lows are often generating steep pressure gradients, always with very significant winds, sea surface circulation is of course counterclockwise. A Question always comes when watching this… Which current gives rise to which water or air current? Since, at that location, there are huge streams with significant temperature differences equally clashing in both mediums, a conclusion would be that both air and water are spinning under the same influences.. If so it is a better concept in understanding water and air circulations.

Excellent post. This idea about “driving”…. does a car analogy work here?

I think I drive my car. But actually, in an energetic sense, it is the engine that does that. However, my steering of the car (“modulating”) is far more important to everyone’s safety! In a plain English sense I do drive the car; and the density patterns do drive the THC? Perhaps “steer” rather than drive would be a better word?

The term ‘thermohaline circulation’ has been criticised, as Gavin states, because it “tends to perpetuate the idea that it is the temperature and salinity fluxes that ‘drive’ the circulation”. On that basis we’d also need to abandon the term ‘steam engine’, as this machine is not driven in an energetic sense by steam, but by coal or wood.

The term ‘thermohaline circulation’ (THC), on the other hand, is a time-honoured and well-established term in physical oceanography since early in the 20th Century, which was coined and used by pioneers of our field like J. Sandström and A. Defant – who clearly knew that the temperature and salinity fluxes do not provide the energy for this type of flow. They were the people who discovered and analysed this in the first place! I would caution against throwing out our scientific heritage for no good reason, and with no better term to replace it.

The term ‘thermohaline circulation’ refers to a particular driving mechanism, which is clearly distinct from the way tides or the directly wind-driven circulation are driven, and in which thermohaline surface fluxes play the distinctive and crucial role – even if they do not provide the energy.

Note also that this energy argument applies only to a special case: the long-term (multi-millennia, equilibrium) circulation in a closed system (i.e. the global ocean). It does not apply to the transient case (i.e., a polynya opening up in ice can drive a thermohaline flow, even in an energetic sense, see Buffoni et al. 2002). It does not apply to the regional case, e.g. the thermohaline circulation between Atlantic and Mediterranean, driven by the fact that the Mediterranean loses about a meter of water each year due to evaporation.

The term MOC has a very different meaning. To paraphrase my recent review paper:Although the terms THC and MOC are often inaccurately used as if synonymous, there strictly is no one-to-one relation between the two. The MOC includes clearly wind-driven parts, namely the Ekman cells (…) On the other hand, the THC is of course not confined to the meridional direction; rather, it is also associated with zonal overturning cells.

Unfortunately, some people have recently started to use the term MOC when they really mean the THC – perhaps they feel there is “something wrong” with using THC, without fully understanding the issue. Rather than abandon the term THC and with it a useful concept and a part of our scientific heritage, my plea would be to be more accurate in using the term. After all, if the great oceanographers of the 20th Century had not invented and used this term to distinguish flows by driving mechanism, we would probably need to invent it now.

Increasing the pressure of water @ 4C has extremely little effect on the temperature because cool liquids are almost incompressible for any pressures found on Earth. I’m guessing that geothermal heating is pretty minor too, as the heat flux is only about 20mW/m^2 and the deep ocean is pretty close to 4C everywhere.

If I understand correctly, things happening near the surface are what drive the deep ocean currents.

Re: 17 This in follow up to my comment concerning Seager’s article in the July-August, 2006 issue of American Scientist, which is based on the paper Seager et al 2002, “Is the Gulf Stream responsible for Europe’s Warm winters”.

Seager article’s in American Scientist challenges the hypothesis that a stoppage of the thermohaline conveyor can cause dramatic drops in the North Atlantic temperature such as the Younger Dryas. (The Younger Dryas occurred 12,900 years ago and was a dramatic interruption of the Holocene interglacial warming, which brought near glacial cold back to the Northern Hemisphere.)

Seager: “For many years, the leading theory for what caused the Younger Dryas was a release of water from glacial Lake Agassiz…(which) flooded into the North Atlantic, it was said, lowering the salinity and density of surface waters enough to prevent them from sinking, thus switching off the conveyor. The North Atlantic Drift then ceased flowing north, and, consequently, the northward transportation of heat in the ocean diminished. The North Atlantic region was then plunged back into near glacial conditions. Or so the prevailing reasoning went.”

“Recently, however, evidence has emerged that the Younger Dryas began long before the breach that allowed flood water… What is more, the temperature changes induced by a shutdown in the conveyor are too small to explain what went on in the Younger Dryas. Some climatologists appeal to a large expansion in sea ice to explain the sever winter cooling. I agree that something of this sort probably happened, but it’s not at all clear to me how stopping the Atlantic conveyor could cause… to bring on this vast change.”

“But from what specialists have long known, I would expect that any slowdown in the thermohaline circulation would have a noticeable but not catastrophic effect on climate. The temperature difference between Europe and Labrador would remain. Temperatures will not even drop to ice-age levels, not even to the levels of the Little Ice Age.”

“All Battisti and I did was put these pieces of evidence together and add a few more illustrative numerical experiments. … Why had these collective studies not already led to the demise of claims in the media and scientific paper alike that the Gulf Stream keeps Europe’s climate just this side of Glaciation? It seems this particular myth has grown to such a massive size that it exerts a great deal of pull on the minds of otherwize discerning people.”

Does Seager provide a fundamental challenge to the hypothesis that the “Thermohaline circulation is the Achilles heel of our climate system”?

[Response: Very nice cutting and pasting! (I would recommend reading the entire article though). Seager takes exception to the notion that the difference between Labrador and Europe is due to the heat transported by the Gulf Stream. Fair enough – this has sometimes been claimed in popular articles on the subject and is wrong. The same is true for the ‘new ice age’ idea should the circulation cease. Neither of those things are being claimed here. However, that is definitely not the same as saying that the ocean transports don’t affect European climate – it just indicates that they affect the North Atlantic relatively uniformly across longitude. It’s true that the trigger for the Younger Dryas remains a mystery, but whatever the trigger, the principle effect was almost certainly an almost complete shutdown in overturning – look at some of the recent work by McManus and colleagues for support of that. The ‘slowdown’ in your next quote refers to something else entirely – the projected changes in the overturning as a function of greenhouse gas forcing, and I would actually go further – it’s highly unlikely that any such slowdown would give any actual cooling in Europe. More likely it would simply locally moderate the global warming. I don’t see Seager’s work as a fundamental challenge to anything other than over-excitable reporters. Like with Wunsch above, it would be nice if people could debunk myths without sowing confusion – that doesn’t seem to be the case here. – gavin]

Seager’s paper in the American Scientist refers back to earlier work presented in 2002. In that paper, Seager et al. compare the results of 2 modeling experiments. The more recent experiment using the NCAR CCM3 is compared with earlier runs with the NASA GISS model, presented in 1999. To understand the latest paper, one must read the 2002 paper, which begins by stating:

“…To test this we performed a pair of experiments with an atmospheric general-circulation model (GCM) coupled to a uniform depth mixedlayer ocean…. In both experiments the sea-ice cover was held Â�fixed at its annual-mean value in order to eliminate feedbacks…”

then further notes:

“The GISS model also includes a thermodynamic sea-ice model so the sea-ice extent was allowed to adjust when the specified OHT was removed…”

and later:

“The remarkable exceptions are that, in the GISS model, where
sea-ice cover is allowed to vary, removal of OHT causes an expansion of winter sea ice near Kamchatka and in the Norwegian and Barents Seas that, in turn, causes dramatic cooling (as much as 20 degC) of the air temperature immediately above and to the east…”

“The more fundamental difference is that, in the GISS model, expansion of sea ice when the OHT is removed causes the surface air north-west and north of Scandinavia to cool by many degC more than in the CCM3 experiment…”

Finally admitting that:

“More problematic for certain regions is the treatment of sea ice. In CCM3 we held the ice cover fixed, but when it was allowed to vary in the GISS model removal of OHT caused a large expansion of seasonal ice cover in the Kamchatka region and in the Norwegian and Barents Seas, cooling the air above and to the east. The thermodynamic sea-ice model in the GISS GCM probably overestimates the increase in sea-ice extent….”

But, how can they be sure that the GISS over estimates the change in sea-ice if they didn’t model changes in sea-ice with their other simulation?? And, the dramatic cooling over Northern Europe might be just the beginning of changes, as they only ran their experiments for 30 years. Surely it is to be expected that changes in sea-ice would be a major consequence of a shutdown of the THC and any attempt to model such a change must include a realistic dynamic sea-ice model. Seager’s work did not perform such an experiment and downplayed the effects found in earlier work. Why then, would anyone accept this latest modeling result using the CCM3?

RE: #22 – I have long had some issues with the Agassiz mechanism. Hudson’s Bay is, in oceanic terms, a backwater. Anything added to it would take quite a while to diffuse north. Also, I think it is not wise to assume that Hudsonian waters solely route into the Davis Straight. Given the impact of the Polar Easterlies, I’d want to investigate waters also heading northwest then west, circulating and diffusing within the Arctic and into the Pacific.

Thermohaline circulation – if one wants to predict the behavior of two adjacent water masses, then the two important variables needed are the temperature and the salinity; for example when Meditteranean (warm but salty) water meets Atlantic water, the tendency is for it to sink; likewise cold salty water produced as sea ice forms will sink as it encounters warmer tropical currents – and ‘currents’ can be hundreds of miles wide – more like bedsheets than like rivers.

The problem with using a word like ‘driven’ is that it implies some kind of structural rigidity – for example, a driven pendulum or a driven pump. If deep water is ‘pulled up’ by wind-driven upwelling, that means that nutrient rich water is rising up, but linking that to water sinking somewhere else is not at all straightforward. A similar problem arises in contemplating El Nino. How do conditions on one side of the Pacific (Indonesia) affect conditions on the other side? What’s the ‘teleconnection’; or how is the energy redistributed from one region to the next? All this begs for more data from the oceans… isn’t NOAA’s budget getting cut?

What if we made a model of the Earth that had no landmasses near the polar regions to help out with cold deep water formation (about 50 million years ago Antarctica wasn’t over the South Pole), would the thermohaline circulation of today no longer exist? Would there be a much warmer ocean bottom, rather than the bone-cold situation of today? Isn’t there a fossil record signature of the ‘refrigeration of Antarctica’ 30 mya – a species change brought on by the cold bottom water? (see #9) This is important – we should ideally be able to predict what the climate of such a world would be like.

In today’s modern world, with increased melting of the Greenland and Antarctic Ice Sheet, who knows what will happen to the circulation without better data and monitoring? Even if the MOC heat transfer decreases, wouldn’t that likely be compensated for by a warmer Gulf Stream and a warmer Atlantic basin? Is there anything at all that argues against an accelerating rate of heat transfer from equatorial to polar regions, and accelerating glacial melt? What will the world’s coastlines look like in 50 years? 100 years?

An equally important question is whether or not economists are willing to look 50 or 100 years into the future – most economists seem to view five years (or less) as the practical limit of their theories.

I agree that the Gulf Stream as it leaves the western side of the North Atlantic is largely a wind driven current. However, I think Gavin has missed a major fact of the THC circulation question. This same point was also not addressed in the earlier Real Climate post and comments in May 2005. Gavin appears to be looking at the ocean as it is now, after the sinking and up-welling has reached a relatively stable state. But, humanity is changing things, which may include changing the basic configuration of the oceans as well as the atmosphere.

As Broecker pointed out several times, there is a net transfer of water from the North Atlantic to the Pacific across the Isthmus of Panama. This process results in a build up of salt in the North Atlantic. There is also a salty flow from the Mediterranean Sea into the North Atlantic via a deep current thru the Strait of Gibraltar. The notion that there is no “driving” force to cause the THC sinking ignores the effect of gravity, which pulls the denser water from the North Atlantic to the bottom of the world’s oceans. There, it accumulates on top of the still colder, saltier waters produced around the Antarctic as the result of the yearly cycle of sea-ice. Drop a rock in the ocean and it WILL go to the bottom.

To be sure, one could say that the THC sinking is “wind driven”, since the prevailing winds cause the fresh water transfer between ocean basins, but that’s not the point. Gavin’s previous point about the North Atlantic circulation being a “closed” system is not correct, as the fresh water transfer shows. There is also a flow of relatively fresh water from the North Pacific into the Arctic Ocean thru the Bering Strait and a flow of fresh water in the form of sea-ice between the Arctic Ocean and the Greenland Sea via the Fram Strait. The water which eventually sinks as the THC is actually fresher than the water in the middle of the North Atlantic, as fresh water is transferred by atmospheric circulation between mid-latitudes and the higher latitudes above 60N. As a result, the sinking waters must become very cold, almost to freezing, in order to sink in the convective “chimneys” reported in the Greenland Sea by Peter Wadhams. This sinking is thus seasonal, being directly the result of very cold winter temperatures . If the water is slightly fresher, it will not sink before the freezing point is reached.

As Gavin mentioned in the earlier post, there is evidence that the THC circulation exhibits variation and even may have stopped in the Greenland Sea during the late 1970’s thru the mid 1980’s, [Schlosser et al. (1)] . This slowdown has been associated with a pulse of fresh water called the Great Salinity Anomaly (GSA). Although Gavin’s conclusion suggests that a slowdown or shutdown of the THC is unlikely in the near future, he appears to ignore what happened during this earlier period, as well as the 8,200 year episode. Studies have shown other GSA type events since, suggesting a process of repeated small floods and there is apparently a freshening of the Nordic seas that is ongoing. At the very least, there is a continuing flow of sea-ice thru the Fram Strait representing about 0.07 SV of fresh water and there is a very recent report of a near breakup in the Arctic sea-ice on the Atlantic side of the Arctic Ocean this summer. Another strong GSA event could occur at any time, perhaps giving another round of very cold winter conditions as seen in the late 1970’s and early 1980’s (2). Recall the hard freezes that destroyed the citrus trees in Florida and Texas and record cold which contributed to the Challenger accident in January 1986.

But, a shutdown of the THC in the areas where it now occurs will not stop the buildup of salt in the North Atlantic. Indeed, that process may be accelerating due to another of mankind’s activities. The damming of the Nile has nearly cut off the flow of fresh water into the Mediterranean Sea (3), thus the waters there are becoming more saline. The water which exits the Strait of Gibraltar is so dense that it slides below the surface layer and can be found for a considerable distance. All this added salt will cause sinking, but the next question would be where would this occur and what would be the consequences.

One possibility, which may be underway presently, is for more warm North Atlantic water to flow into the Arctic. There have been reports that the intrusion of Atlantic water into the Arctic is now progressing further toward the North Pole than previously seen. One consequence may be increased melting of the sea-ice over the Arctic Ocean, as melting from below is enhanced. The present trends in maximum and minimum extent of sea-ice imply a greater area of yearly melt and freeze of Arctic sea-ice, which would tend to produce more salty brine that would sink to the bottom of the Arctic, much like the process around the Antarctic. The resulting salty water would add to that which exits along the bottom of the Fram Strait, thus adding to the flow of water over the Greenland-Iceland-Scotland sill. If this were to occur, the location of the sea-ice edge might shift, with more sea-ice seen over the Sub Polar Gyre and less in the Barents Sea. If present trends continue, eventually all the sea-ice will melt by the end of the summer season.

One further possible result could be a buildup of salt in the Sargasso Sea to a level such that sinking begins to occur there. After all, we know that the Mediterranean outflow is quite dense. I see no reason to think this water might not continue to the bottom, were it to become salty enough as the Earth warms and the P/E ratio changes over the Mediterranean drainage. The
results of sinking warm water could ultimately change the entire planet in ways which can only be imagined.

After reading the article and the comments, it would seem to this layman that Western Europe will be habitable for the forseeable future. No need to sell the house just yet.
It also seems that we need a few hundred years more of detailed data before we get a really good picture of how this planet works. By then, on the other hand, it might be too late. So reducing the greenhouse gases seems to be a good idea at any rate, even if it takes an erroneous doomsday scenario or two to make things happen.